The present invention relates to a diagnosis apparatus for a fuel vapor purge system, which supplies fuel vapor in a fuel tank to an intake system of an internal combustion engine.
Fuel vapor purge systems for sending fuel vapor in a fuel tank to an intake passage have been proposed. A typical fuel vapor purge system includes a canister, a vapor passage for connecting a fuel tank with the canister and a purge line for connecting the canister with an intake passage. The canister has an atmosphere valve through which the canister is exposed to the atmosphere. Fuel vapor in the fuel tank is collected by the canister. The collected fuel vapor is supplied to the intake passage through the purge line. A purge valve is located in the purge line to control the amount of fuel vapor supplied to the intake passage from the canister.
For example, Japanese Unexamined Patent Publication No. 4-362264 discloses a diagnosis apparatus for detecting leakage of fuel vapor through a puncture or a crack from a fuel vapor purge system. The diagnosis apparatus temporarily maintains a vacuum pressure in the purge system, or a pressure that is lower than atmospheric pressure. Then, the diagnosis apparatus observes changes of the purge system pressure over time thereby detecting whether there is a leak.
It is desirable that the diagnosis apparatus be able to quickly and accurately detect leakage through minute holes and cracks. However, the prior art diagnosis apparatuses cannot detect leakage through holes having a diameter that is smaller than 1.0 mm. Future regulations against pollution are likely to require that extremely small amount of vapor leakage be detected. Therefore, there is an increased demand for a diagnosis apparatus that detects holes smaller than 0.5 mm in diameter.
The diagnosis apparatus of Publication No. 4-362264 accurately detects vapor leakage only for a short period, for example, immediately after the engine is started. Further, when the amount of fuel in the fuel tank changes, the vapor pressure of the fuel changes the pressure in the purge system, which may cause the diagnosis apparatus to obtain erroneous diagnosis results.
Accordingly, it is a first objective of the present invention to provide a diagnosis apparatus that accurately and quickly detects fuel vapor leakage from a fuel vapor purge system. A second objective of the present invention to provide a diagnosis apparatus that frequently performs diagnosis.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, this invention provides a diagnosis apparatus for a fuel vapor purge system. The purge system includes a fuel tank for storing fuel and supplies fuel vapor from the tank to an air-intake passage of an engine. The diagnosis apparatus determines whether the purge system has a malfunction. The apparatus includes a pressure sensor, a pressure changing means, and a diagnosis means. The pressure sensor detects the pressure in the purge system. The pressure changing means changes the purge system pressure to a predetermined level. The diagnosis means diagnoses the fuel vapor purge system. The diagnosis means closes the fuel vapor purge system after the purge system pressure has been changed by the operation of the pressure changing means. The diagnosis means measures a first rate of pressure change when the purge system pressure approaches a predetermined first reference pressure. The diagnosis means measures a second rate of pressure change when the purge system pressure approaches a predetermined second reference pressure. The second reference pressure differs from the first reference pressure, and the second reference pressure value is closer to the pressure of the purge system before the pressure of the purge system was changed by the pressure changing means than the first reference pressure. The diagnosis means judges whether the purge system has a malfunction based on the ratio of the first rate to the second rate.
This invention further provides a method for diagnosing whether a fuel vapor purge system has a malfunction. The purge system includes a fuel tank for storing fuel and supplies fuel vapor from the tank to an air-intake passage of an engine. The method includes changing the pressure in the purge system to a predetermined level, closing the purge system after the purge system pressure reaches the first pressure value, measuring a first rate of pressure change at a first reference pressure, measuring a second rate of pressure change at a predetermined second reference pressure that differs from the first reference pressure, and that is closer to the pressure of the purge system before the pressure of the purge system was changed to the predetermined level than the first reference pressure, and calculating a ratio of the first rate of pressure change to the second rate of pressure change.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.